In metal sheet processes, lubricants are often used to facilitate cutting, stamping, bending, drawing, ironing and other forming operations required to convert a work piece into the desired product, while offering low tool wear and build up.
In the United States, some lubricant formulations used to form food, beer and beverage containers and parts of thereof must comply with the United States Food and Drug Administration (FDA) Regulation No. 21CFR178.3910.
Conventionally, kerosene-based lubricants have been used in sheet processing. However these lubricants tend to cause sticking of chaff formed during conversion operations. As well, evaporation of volatile oil lubricants contributes to the formation of volatile organic compounds (VOCs) and poses health and environmental problems.
Presently, two semi-solid pre-lubricants are commonly used as tabstock lubricants. One formulation is described in U.S. Pat. No. 5,672,401 by Alcoa, filed in 1995, herein incorporated by reference. Another material, produced by Force Industries and sold under the name AMCO 4942™, is also used. However, these lubricants have shown poor adaptability to different modes of application and work conditions.
Often, sheet metal products, such as can body sheet, are pre-lubricated prior to forming and/or post-lubricated with liquid lubricants after rolling. These liquid lubricants tend to flow off of the sheet surface during processing, or seep from the coiled sheet metal in storage and transportation, due to the generally low viscosity and/or poor wettability. To overcome this, excessive amounts of lubricant are often applied, leading to additional cost and waste.
Other lubricants, such as those composed of mono- and dilaurate esters of ethylene glycol, often used in pre-lubricated automotive sheet, tend to be overly brittle waxy solids at room temperature and tend to detach themselves from the sheet during transportation or processing.
In sheet rolling, traditional lubricant components such as fatty alcohols, fatty acids or fatty acid monoesters, such as methyl and butyl, have not always been successful in preventing strip breaks that occur due to the load of work rolls of a rolling mill and result in product quality problems.
Consequently, lubricant formula selection, and also lubricant application methods are crucial to successful use in the production of metallic articles, including food, beer and beverage containers.
At present, surface lubricants formulated in agreement with FDA Regulation 21CFR178.3910 are typically applied using roller coater or other mechanical techniques, which do not always ensure uniform surface coverage and often lead to excess deposition of lubricants on the articles.
One method that has been found to result in uniform application of lubricant is electrostatic application. The electrostatic application of surface lubricants is economically and technically desirable because it can be done at high line speeds and at precisely controllable application levels. Furthermore, because very little excess of lubricant is used, this technology allows easy switchover from one lubricant to another, and leaves a relatively small environmental imprint.
However, lubricants must have a certain level of conductivity to be applied using electrostatic applicators. Hydrocarbon and fatty acid ester or fatty acid based lubricants most commonly used in food, beer and beverage applications and formulated in accordance with FDA Regulation 21CFR178.3910 are not conductive and require conductivity enhancers in order to make them amenable to electrostatic techniques.
There are only a limited number of conductivity enhancers that are soluble in hydrocarbon-based lubricant formulations. Of these, lecithin is one of the only materials that is used commercially as a conductivity enhancer for surface lubricants, and which also meets necessary FDA requirements. However, its presence in lubricants may compromise the taste and odour of food, beer or beverages packaged in metallic containers lubricated with such lubricants.
U.S. Pat. No. 5,135,669 discloses the use of hydrogenated lecithin as a friction-modifying agent for fuels and lubricating oils, but only discusses lubricating oils for use in automatic transmissions of vehicles.
U.S. Pat. No. 2,295,192 also discloses hydrogenated lecithin as an additive for lubricating oils.
Finally, U.S. Pat. No. 6,207,286 and U.S. 2002/0006519 disclose a metal sheet product formed from food container stock. The lubricant composition contains natural lecithin as a conductivity enhancer.
It is therefore greatly desired to develop lubricants for use in the production of sheet metal products, which are safe for use on food, beer and beverage containers and provide suitable properties for metal processing. It is also desirable to find additives to lubricants that will enhance conductivity of the lubricants for electrostatic application, but which will also be substantially odour and flavour-neutral.